I looked at the shed roof some more, and there is barely room to fit a
false ceiling beneath it, with a layer of fiberglass insulation and
Tyvek housewrap outside that. I say that because every further inch of
space would take away light from the shuttered, 16" tall, 7' wide
Solacryl UV-transmitting window at the top of the south wall. The
interior walls and false floor are less constrained.
I examined the material they put between the metal roof and the
purlins. It feels like plastic canvas. But that certainly doesn't
exempt us from dealing with internal condensation, since moisture would
now just collect on the plastic! However it does protect the purlins
from moisture that might seep through the unsealed roof. It will also
keep water vapor from cooling quite as rapidly as it would if it struck
the metal directly.
Thanks to that plastic layer, the purlins do indeed create separate
"boxes" in which water vapor could be trapped. But the plastic is
somewhat loose, so we could maybe insert some small plastic tubes here
and there to allow water vapor to flow uphill against the plastic and
into the next box (the idea being to get away with just a single vent
hole). Then maybe we'll just cover the small rectangular opening, where
the exhaust air flows out along the center spine, with a screen on one
end. Thanks to the warmth inside, the air flow should consistently be
upward, so if there's no draft in the roof space from cross-ventilation,
the air space should retain some warmth, and yet still be open at one
end (especially since the opening is well under a good eave).